1. Field of the Invention
The present invention relates to a projection lens, and more particularly to a projection lens used in a video projector for obtaining an enlarged picture on a screen by projecting an image appearing on a cathode ray tube (CRT).
2. Description of the Prior Art
U.S. Pat. No. 4,300,817 or U.S. Pat. No. 4,348,081 shows a basic arrangement of a projection lens, which comprises three lens units (groups). However, correction of chromatic aberration is not considered in these U.S. Patents.
A projection lens used in a video projector of the three-tube type is required for obtaining a high quality color image to have a capability of correcting chromatic aberration. A projection lens having the chromatic aberration correction capability is disclosed Japanese Laid-Open Patent Application No. 58-198016. This lens is composed of, from the screen end, a first convex lens element of bi-convex type, a meniscus convex lens elements having a concave surface directed to the screen end, and a second concave lens element. The first, second and meniscus convex lens element is made so as to have an Abbe number of 50 to 65 and the first concave lens element is made so as to have an Abbe number of 20 to 40, thereby reducing the chromatic aberration. The lens elements except for the second concave lens element are made of plastic.
Another projection lens having the chromatic aberration correction capability is disclosed in Japanese Laid-Open Patent Application No. 58-139111 or its corresponding U.S. Pat. No. 4,530,575. This lens comprises, from the screen end, a first positive lens element having a convex surface directed to the screen end, a second positive lens element, a third negative lens element, a fourth biconvex lens element made of a glass material, and a fifth negative lens element. The lens satisfies specific conditions to correct chromatic aberration.